Originally posted by Edgar_in_Indy Even with the K-5, I still wouldn't want to give up f/2.8. Faster shutter speeds and lower ISO are always a good thing, as well as having more control over DOF. So I, personally, would take the constant f/2.8 over the extra 20mm.
That's an interesting bit of information that the 17-50mm and the 17-70mm use the same optical formula. Looking at the two lenses on Sigma's website, I see that they both specify 17 elements in 13 groups. Although the 17-50mm says it uses FLD glass, while the 17-70mm uses ELD glass. FLD is supposed to be superior to ELD, so it appears that the optical formula is not EXACTLY the same, but may be very similar. This is from Sigma's website:
FLD ("F" Low Dispersion) glass is the highest level low dispersion glass available with extremely high light transmission. This glass has a performance equal to flourite glass which has a low refractive index and low dispersion compared to current optical glass. FLD glass offers superior optical performance, equal to flourite, at an affordable price.
ELD (Extraordinary Low Dispersion) glass has lower dispersion characteristics than SLD (Special Low Dispersion) glass, which Sigma has been using in its APO lenses (and some non-APO lenses as well) for many years now. It has other advantageous properties as well.
My only question is, when I can buy a Sigma 17-70mm f/2.8 HSM?
That would take care of the constant inner conflict I have about selling my Tamron 28-75mm f/2.8 to replace it with the Sigma 17-50mm f/2.8.
oh well, the tech gentleman said there is not much noticeable difference between ELD and FLD glass in terms of sharpness as we do test in Sigma lab. I even called back again at the later time to talk to another tech and she pretty much said the same thing. They both said EX lenses pretty much use the same optics as DC lenses and the only advantage is constant aperture...
Since I mostly shoot outdoor and usually with higher f# ( street, landscape ) during the day, he said there won't be any difference in IQ on either lens. As far as indoor or studio shooting, he said as long as I have external flash or soft box etc, which I do, and set the aperture to 8, again there won't be any difference on IQ with either lens. The only time I would notice the difference is if I want to shoot indoors ( house, church etc ) without the flash and raise the ISO with 17-70 vs keep the ISO low with 17-50 because of f2.8;then, there will be significant difference.
He also highly praised 17-70's macro capability compare to 17-50 which is not macro.
Since you have lot more experience in the field, for general walk around, Landscape, street photo with K-5 what do you recommend my friend ? ( I also got Tamron 28-75 for portrait as you recommended )
Thanks,